The ability to form electronic circuits on low-cost substrate materials (e.g., “plastics”) allows a substantial cost reduction, as compared to processes using more expensive substrates, such as silicon or electronic-grade glass. In order to obtain such a cost-reduction advantage, it is desirable to realize high-performance electronic devices (e.g., transistors), using processes with the low-cost substrate materials. A typical example of an application benefiting from such high-performance devices is a backplane for a display.
Typically, candidate low-cost substrate materials (e.g., organics and inexpensive glass) are also quite temperature-sensitive. Thus, it is necessary that high performance electronic devices be fabricated using processes that do not expose the substrate material to high temperatures. Typically, it is desirable to use processing temperatures that do not exceed the range of about 100° C. to about 300° C. Lower temperatures typically enable the use of lower-cost substrate materials. As an example, various organic semiconductors and dielectrics can be used to build transistors at quite low temperatures, compatible with low-temperature inexpensive substrate materials. Furthermore, organic semiconductors generally exhibit good mechanical flexibility, important for integration with flexible substrate materials. However, due to intrinsic material limitations, organic semiconductors in general show quite low mobilities (e.g., about 1 cm2/Volt-sec or even much lower), which reduce their applicability to high-performance, large-area applications such as displays. Low mobility similarly limits applicability of amorphous silicon. While organic thin-film transistors (TFT's) may in some instances be suitable for pixel-switching elements (e.g., for bi-stable display technologies), they are generally not suitable for on-glass drivers or other processing circuitry, which can be integrated to further decrease overall display cost. Plastic substrates, besides having lower costs than other substrates, also provide benefits of flexibility, shock resistance, and light weight.
Another approach involves the use of more conventional inorganic semiconductors, such as silicon, which require high processing temperatures. High-temperature processes are carried out prior to a transfer step in which processed semiconductor nanowires, nanoribbons, or other such structures are transferred to the low-cost substrate material. For example, in the paper “High-performance thin-film transistors using semiconductor nanowires and nanoribbons,” Nature, v. 425, (18 Sep. 2003) pp. 274-278, X. Duan et al. have reported mobilities above 100 cm2/Volt-sec for silicon nanowire TFT's on plastic, using such an approach. While such an approach enables high mobilities and excellent performance, process complexity is increased, depending on the nature of the transfer process, so that the substrate-related cost reduction may be counteracted to some degree by costs associated with added process complexity.
It is also possible to directly deposit and process certain inorganic semiconductors on low-temperature substrate materials. Amorphous silicon TFT's can be directly processed at temperatures compatible with “plastic” substrate materials. For example, mobilities of about 0.4 cm2/Volt-sec have been reported with a maximum processing temperature of about 180° C. (S. H. Won et al., IEEE Electron Device Letters, v. 25, n. 3, (March 2004), pp. 132-134). Amorphous silicon, however, has similar mobility limitations and associated limited applicability as described above for organic TFT's. Polycrystalline silicon TFT's can also be directly processed on “plastic”, using laser crystallization; for example, mobilities of about 65 cm2/Volt-sec have been reported with maximum processing temperature of 320° C. (F. Lemmi et al., IEEE Electron Device Letters, v. 25, n. 7, (July 2004), pp. 486-488).
Previously, a mobility of 60 cm2/Volt-sec for polysilicon TFT's had been reported with a maximum processing temperature of 150° C., also using laser crystallization: S. D. Theiss et al., “Polysilicon Thin Film Transistors Fabricated at 100° C. on a Flexible Plastic Substrate,” IEDM Technical Digest (1998), pp. 257-260. Potential drawbacks for such approaches are added cost and complexity associated with laser crystallization.
While all of these prior methods have resulted in useful devices, additional thin-film devices and methods for their fabrication are needed limitations of low performance, process complexity, and/or processing temperature.